What is the minimum a comm can be cut to? Is there a sweet spot.


Thanks.

the motor itself will decide its own sweet spot...

as for comm size i wouldnt go lower then .265 or even .270 i had one shred on the lathe this weekend, and it was at .272.. made one light skim then boom...

a motor will look good on the dyno when the comm is small. but from my expirience, they are slugs on the track,,,

I have had em set track records at anywhere from 2.65-2.90
Like Burbs said some motors like small coms, some like larger.
Some motors will run under 2.70 some won't.
Some motors run at 2.90 some won't.

I have seen exceptions, but typically a new motor has a bit more torque and lower RPM with a Big com. Then as you cut it, power and RPM seem to increase and you lose a bit of Torque. The point where all 3 numbers (Watts, RPM and Torque) seem the best on the dyno is about 2.78-2.85 range. They usually hump on the track at that point too. So for me if I had to pick a sweet spot I would say 2.78-2.82 personally for a Monster motor. Now with the Epic Roar stocks, I like the com's a little bigger 2.82-2.88

I haven't kept that close of a record to the actual size, but mainly how much I was taking off. From what I do remember though seems like they were around .277-.270.
I have noticed the fall off with the more cuts, I didn't now if it was motor or batteries, but the #'s on the batteries are where they was last year. I run the #'s about every three months or so.

Thanks guys.

When you mean "fall off", are you talking lap times are falling off at the end of the run?

If so that is usually the sign of something inefficient with the car.
Voltage up front can mask an ill handling car but once that peak comes off then you see lap times fall quick.

That or maybe your gearing is slightly too high.

Lap times falling off shouldnt have much to do with com size.

I've run the same gear for a while, give or take a tooth. Laps times are about the same.
Never gets the motor to hot like it would be over geared. Everything is free.

The charger could be misleading me with the numbers.

Could be
Plus I have had packs too that still cycle well but are older, and they dont run like they once did too, so consider that.

I think in a theroretical sense, the size of the comm is significant factor in terms of how well the motor is capable of running. There may be some frictional issues that could prove to be dependent on comm size, however I think they are probably pretty tiny and most likely do not play a significant role, perticularly if your talking about a standard size comm as oposed to ha larger then standard comm...

If there is some other advantages to larger or smaller, it would relate to timing and/or duration of the powering of the electro magnetc feilds on the arm... these factors are just as effected by brush width as they are by the comm size. The width of the slots between comm segments also plays a role in these issues. Then there are also issues with how consistantly (uniform distances) the comm slots are cut in relation to one another and even in relation to the location of the actual phsical segments where the windings are...

In general... I beleive that you can play with the width of the brush to compensate for the size of the comm, and if all other factors are aligned with the sun and moon, you can make any size comm work on any given model of motor...

Hows that for a answer? :)

Thats what I thought was happening.......LOL. And its getting close to a full moon.

As the comm gets smaller, should I go to a narrow brush? I never thought about that.
How many runs do ya'll average with a motor before replacing?

Dyno, you a politician ?:devil:

I have found if you take a small comm armature and put it in a fresh can and endbell it more often time then not a fast motor ....... go figure

the new trinity cobalt has an explaination paper that says someting about the brush vs com size....too much brush/ segmet overlap is not good...

From my experience it really doesn't matter what size the comm is to go fast. I have heard from some of the top stock drivers that if you have a small comm motor to save it for the main. Its not uncommon to see a brand new handout motor at big races get cut down to nothing right from the start. I think alot of the performance comes from the actual assembly of the motor. You can look at 10 different motors of the same kind and all will have different dyno #s and only a few of the 10 will be better than the rest on the track. I've also had motors run good on a cut, then run bad on the next and back to good after that. I've also heard that too much brush wrap can short out the segments. The TQ motor at the birds had slightly timed brushes but I don't know what the exact comm size was though.

The smaller the comm the more brush overlap. The same is true for the width of the comm slots, smaller means more overlap... Some people and some motors like haveing maximum overlap, the most overlap you could get would be with full widith laydown brushes and the smallest comm, with the narrowest comm slots...

I personaly think that most modern (high RPM) stock motors have more then enough brush overlap even when the comm is full sized and the brushes are standard widith. Many good stock motors get built these days with their brushes narrowed... Todd Putnam regularly builds his motors with narrowed brushes, and well Todd typicaly can get very good results this way... (as have I, but I don't have any TQs or wins at national events). Todd typicaly cuts the trailing edge of the brush back...(as do I)...

I honestly don't know what Todd's posistion on the actualy size of the commutor is, and I don't mean to imply that his opions are one and the same with mine... Just that maximum overlap is clearly not nessasary with most modern motors, I'm using Todd's practices to back up my own personal observations with regard to brush width... I have no connection at all with Todd or any other motor builders.

Bottom line for me is you generaly can make any size comm work well if you understand what's really going on, and how it all relates to the actual mechanical/electrical issues involved in making a motor run fast and effcient. Power at one perticularly point on a dyno's output curve does not always mean you're motor will out proform someone elses with a differnt power curve. There are many other factors such as effciency and even in the motors ablity to disapate heat effectively. It's almost impossible to totaly determin a motors proformance based soley on dyno data.
(or amp draw at no load for that matter).

After a couple cuts, would anyone recommend narrowing the brush a couple thousandths or just the trailing edge?

Thanks.
_______________
This is good stuff

.260 is where you run out of copper on most arms (at least that's where I've run out :freak: ). The smaller the arm, it seems, the higher the rpm but with a trade off of power (ie. bigger comm more power, less r's). As far as a sweet spot, I'm not sure if it has to do solely with comm size. I would imagine its a combination of size as it relates to brush size and other factors. I'm no pro...just my two cents.

.260? WOW!!!! How many times have you done this? I've always stopped at .270. I'm afraid of it blowing apart on me while cutting it. If it blows apart on the track its ok cause it makes for a good show. Sometimes. What kind of arms have you gotten that low? I have a monster at .269 that I was thinking was going to be finished.

I cut down an old arm just to see where the copper ended. Its kind of cool when you hit the bottom because flakes of copper come off but not all the way around (comm isn't true). I was just wondering one day (sort of like this thread) and decided to find out. LOL.

I personaly think that most modern (high RPM) stock motors have more then enough brush overlap even when the comm is full sized and the brushes are standard widith. Many good stock motors get built these days with their brushes narrowed... Todd Putnam regularly builds his motors with narrowed brushes, and well Todd typicaly can get very good results this way... (as have I, but I don't have any TQs or wins at national events). Todd typicaly cuts the trailing edge of the brush back...(as do I)...

I would like to add to this......
I tried this with the narrowed brushes Friday night at the local track, and it really works good. I removed about .020 from the trailing side of the brush and without the use of a dyno, I geared three teeth smaller and it was really quick. Might could have went four teeth, but I think anything smaller would have been to much.
Thanks to everyone that has responded to this and for all the info.

Tommy :thumbsup: